Self-repair integrated circuit and repair method

ABSTRACT

A method for repairing degraded field effect transistors includes forward biasing PN junctions of one of a source and a drain of a field effect transistor (FET), and a body of the FET. Charge is injected from a substrate to a gate region to neutralize charge in the gate region. The method is applicable to CMOS devices. Repair circuits are disclosed for implementing the repairs.

RELATED APPLICATION INFORMATION

This application is a Divisional application of allowed U.S. patentapplication Ser. No. 13/288,472 filed on Nov. 3, 2011, which is aDivisional application of U.S. Pat. No. 8,098,536, issued Jan. 17, 2012,both of which are incorporated herein by reference in their entirety.

BACKGROUND

1. Technical Field

The present invention relates to integrated circuits, and moreparticularly to circuit self-repair to fully or partially recoverdamaged devices.

2. Description of the Related Art

As complementary metal oxide semiconductor (CMOS) technology generationsadvance into submicron and nanometer scale, CMOS-devicethreshold-voltage instability has become a major reliability problem.The threshold voltage (Vt) instability not only reduces the circuit'soperation lifetime, but also adversely affects circuit yields. Forexample, in static random access memory (SRAM) fails occur duringburn-in because of corresponding P-type field effect transistor (PFET)threshold degradation. In analog circuits, severe Vt mismatch couldresult in circuit failure.

It is known that one major cause of the threshold instability of PFETsis due to an effect called negative bias temperature instability (NBTI).NBTI has been widely investigated because it increases PFET thresholdvoltage and decreases the drive current due to the buildup of positivecharge and surface states in the gate dielectric. As gate dielectricthickness is further reduced and new gate materials are employed, NBTIis becoming a more prominent degradation mechanism in PFET devices.Also, NBTI has become the major reliability issue at the circuit-leveldue to its large duty cycle under a relative bias between gate and drainin a specific waveform during circuit operation. NBTI is alsoindependent of device channel length.

The analogous threshold instability of an N-type field effect transistor(NFET) is positive bias temperature instability (PBTI). Note thatcompared with NBTI and other device degradation mechanisms like hotelectron wear-out effect, PBTI is less significant for traditional gateoxide devices. However, as high-dielectric constant (high-k) materialsare introduced as the gate dielectric for advanced technologies, PBTIeffects have a greater impact on the circuit and have to be taken intoconsideration during the course of process development.

Some degraded devices as a result of certain wear-out mechanisms can berecovered. For example, U.S. Pat. No. 4,238,694 discloses a method forrecovering selected areas of a radiation damaged semiconductor by a hightemperature bake. For NBTI, various methods have been practiced in theindustry to minimize its detrimental effect on device performance duringfabrication including tuning a thermal anneal time and hydrogen flow.

Device structures have also been used to reduce NBTI. For example, U.S.Pat. No. 7,030,498, titled “Semiconductor Device with Copper WiringsHaving Improved Negative Bias Temperature Instability (NBTI)”, teachesthe use of a diffusion barrier, such as silicon carbide (SiC), tosuppress NBTI-induced threshold shifting. In U.S. Pat. No. 7,030,498, aPFET structure is disclosed, which includes a nitrogen-containingsilicon oxide and a copper wiring pattern including an underlyingbarrier layer and a SiC layer covering the copper wiring pattern. ThisPFET structure attempts to suppress NBTI deterioration. This fixdeviates from normal processing steps and requires new material which isnot desirable from a cost point of view.

After device fabrication and stresses under NBTI conditions, thermalannealing was observed to partially recover the NBTI degradation. U.S.Pat. No. 6,958,621, entitled, “Method and Circuit for Element WearoutRecovery,” proposes a structure and methodology for circuit recoveryafter NBTI degradation. U.S. Pat. No. 6,958,621 utilizes an annealingeffect for NBTI recovery by employing polysilicon heaters adjacent tothe gates of critical devices. By powering up the heater, the channeltemperature of the critical devices is raised to an annealing level sothat the NBTI-induced interface damage can be partially removed. Thedrawbacks of this method include: (1) a large area is required foraccommodating the heater components, (2) a slow thermal repairingprocess is needed, and (3) a large amount of power is consumed to reachthe annealing condition.

SUMMARY

A method for repairing degraded field effect transistors includesforward biasing PN junctions of one of a source and a drain of a fieldeffect transistor (FET), and a body of the FET. Charge is injected froma substrate to a gate region to neutralize charge in the gate region.The method is applicable to CMOS devices. Repair circuits are disclosedfor implementing the repairs.

A method for repairing field effect transistors in a memory circuithaving an array of memory cells includes, for pull-up P-type devices,switching a ground line from a ground potential to a supply potential;switching on all wordlines in the array; repairing P-type devices on afirst side by connecting an n-well of the P-type devices to a voltageless than the supply potential, but greater than the ground potential;activating bitlines on the first side; repairing P-type devices on asecond side by connecting the n-well of the P-type devices to thevoltage less than the supply potential, but greater than the groundpotential; and activating bitlines on the second side.

Another method for repairing field effect transistors in a memorycircuit having an array of memory cells includes, for pull-down N-typedevices, switching a supply line from a supply potential to a groundpotential; switching on all wordlines in the array; repairing N-typedevices on a first side by connecting a P-well of the N-type devices toa voltage greater than the ground potential, but less than the supplypotential; deactivating bitlines on the first side; repairing N-typedevices on a second side by connecting the P-well of the N-type devicesto the voltage greater than the ground potential, but less than thesupply potential; and deactivating bitlines on the second side.

A memory circuit includes an array of memory cells having a plurality ofaccess transistors controlled and accessed through associated wordlinesand bitlines, the array of memory cells including a supply voltage and aground. A logic circuit is coupled between the array and the supplyvoltage and the ground such that the array is selectively connected tothe supply voltage and the ground in accordance with at least one fixcontrol signal. A fix control circuit is coupled to the logic circuit,the fix control circuit configured to output the at least one fixcontrol signal to enable portions of the array to be connected to atleast one of substrate wells, the supply voltage and the ground torepair field effect transistors by charge neutralization.

The logic circuit may be configured to: for pull-up P-type devices,switch a ground line from a ground potential to a supply potential,switch on all the wordlines in the array, repair P-type devices on afirst side by connecting an n-well of the P-type devices to a voltageless than the supply potential, but greater than the ground potential,activate bitlines on the first side, repair P-type devices on a secondside by connecting the n-well of the P-type devices to the voltage lessthan the supply potential, but greater than the ground potential, andactivate bitlines on the second side. The supply potential is Vdd andthe voltage less than the supply potential, but greater than the groundpotential is Vdd−Vfw, where Vfw is a forward bias voltage. The fixcircuit may include a sensor configured to trigger a fix mode to outputthe at least one fix control signal when a threshold voltage exceeds areference level in at least one device to be repaired.

The logic circuit may be configured to: for pull-down N-type devices,switch a supply line from a supply potential to a ground potential,switch on all wordlines in the array, repair N-type devices on a firstside by connecting a P-well of the N-type devices to a voltage greaterthan the ground potential, but less than the supply potential,deactivate bitlines on the first side, repair N-type devices on a secondside by connecting the P-well of the N-type devices to the voltagegreater than the ground potential, but less than the supply potential,and deactivate bitlines on the second side. The ground potential is Vssand the voltage greater than the ground potential, but less than thesupply potential is Vss+Vfw, where Vfw is a forward bias voltage.

Another method for repairing degraded field effect transistors withfloating bodies includes biasing a gate of field effect transistors(FET) into an accumulating mode (for P-type transistors, biasing gate todrain and/or source nodes to a positive potential, for N-typetransistors, biasing gate to drain and/or source nodes to a negativepotential), and accumulating charge in a gate region to neutralizecharge in the gate region.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a graph of experimental data showing a threshold shift of aPFET device under different bias conditions;

FIG. 2 shows characteristic I-V curves of a PFET device before and afterNBTI stress, and after a recovery process, also depicted in the insetsare transistors showing relevant voltages in accordance with the presentprinciples;

FIG. 3 is a graph of experimental data showing a threshold shift of aPFET device and recovery time behavior, comparing between two biasconditions in a recovery mode;

FIG. 4A is a cross-sectional view of a PFET device;

FIG. 4B is a cross-sectional view of an NFET device;

FIG. 5 is a schematic diagram of a CMOS circuit element;

FIG. 6 is a timing diagram showing waveforms during a recovery mode inaccordance with an illustrative embodiment;

FIG. 7 is a schematic diagram showing a novel SRAM cell including SRAMstructures and a repair structure in accordance with the presentprinciples;

FIG. 8 is a simplified schematic view of one SRAM cell with repairingcapability;

FIG. 9 is a schematic diagram of a 4×4 SRAM array employing repairingfeatures in accordance with one illustrative embodiment;

FIG. 10 is a schematic diagram showing a fix control circuit in greaterdetail in accordance with one illustrative embodiment;

FIG. 11 shows characteristic I-V curves of a PFET device before andafter NBTI stress, and after a recovery process, also depicted in theinsets are transistors showing relevant voltages in accordance with thepresent principles for a floating body device repair;

FIG. 12 is a schematic diagram of a CMOS circuit element with no bodycontacts;

FIG. 13 is a timing diagram showing waveforms during a repair mode inaccordance with an illustrative embodiment; and

FIG. 14 is a schematic diagram showing a novel SRAM cell including SRAMstructures and a repair structure for a device without body contacts inaccordance with the present principles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments in accordance with the present principles provide a recoverymethod employing electron (or hole) injection, to fully or partiallyrecover device damage caused by negative bias temperature instability(NBTI) or positive bias temperature instability (PBTI). One embodimentapplies the repairing method to memory circuits.

A repairing circuit is coupled to a circuit and/or a circuit elementsensitive to a NBTI (or PBTI) wearout mechanism and performs damagerecovery of the circuit with a damaged device. In one embodiment, asensing device monitors the NBTI (or PBTI) wearout mechanism, where thesensing device is employed as a failure monitor to call for repairs. Thesensing device may also be used as a monitor during the repairs tomeasure a repairing period.

Although embodiments will be described in terms of memory circuits thepresent invention is applicable to any number of semiconductor devicesand components and should not be construed as limited by theillustrative examples presented herein. For example, the presentprinciples are applicable to transistors or other devices on a processorchip, memory chip, application specific chip, etc.

Embodiments of the present invention can take the form of an entirelyhardware embodiment, an entirely software embodiment or an embodimentincluding both hardware and software elements. In a preferredembodiment, the present invention is implemented in hardware on anintegrated circuit device however; the device may run software, bedesigned in software or have its design tested in software. The softwaremay include but is not limited to firmware, resident software,microcode, etc.

Furthermore, embodiments of the invention can take the form of acomputer program product accessible from a computer-usable orcomputer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system. For thepurposes of this description, a computer-usable or computer readablemedium can be any apparatus that may include, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device. The medium can be anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system (or apparatus or device) or a propagation medium.Examples of a computer-readable medium include a semiconductor or solidstate memory, magnetic tape, a removable computer diskette, a randomaccess memory (RAM), a read-only memory (ROM), a rigid magnetic disk andan optical disk. Current examples of optical disks include compactdisk—read only memory (CD-ROM), compact disk-read/write (CD-R/W) andDVD.

A data processing system suitable for storing and/or executing programcode may include at least one processor coupled directly or indirectlyto memory elements through a system bus. The memory elements can includelocal memory employed during actual execution of the program code, bulkstorage, and cache memories which provide temporary storage of at leastsome program code to reduce the number of times code is retrieved frombulk storage during execution. Input/output or I/O devices (includingbut not limited to keyboards, displays, pointing devices, etc.) may becoupled to the system either directly or through intervening I/Ocontrollers.

Network adapters may also be coupled to the system to enable the dataprocessing system to become coupled to other data processing systems orremote printers or storage devices through intervening private or publicnetworks. Modems, cable modem and Ethernet cards are just a few of thecurrently available types of network adapters.

The circuits described herein may be part of the design for anintegrated circuit chip. The chip design is created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner transmits the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., Graphic Data System II (GDSII)) for the fabrication ofphotolithographic masks, which typically include multiple copies of thechip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed. Themethods described herein may be used in the fabrication of integratedcircuit chips.

The resulting integrated circuit chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

In accordance with the present principles, a recovery mechanism calledcharge neutralization (herein referred to as CN) may be employed torepair damaged circuit components. Note that most of NBTI degradation iscaused by positive charge trapped at a channel/gate dielectric interfaceunder gate bias. If negative electrons can be injected into the affectedinterface, the trapped positive charge can be neutralized and thethreshold shift by the trapped charge can be recovered. There areseveral ways to inject electrons into a PFET channel for CN recovery.For example, gate tunneling current can be applied for CN recovery.However, this method needs a large voltage drop across gate dielectricand may damage the gate dielectric integrity. Another example employs achannel impact ionization effect, e.g., by stressing a PFET devicehaving a sub-threshold voltage range with a high voltage (e.g., −1.5times Vdd) over a drain and a source while biasing a gate belowthreshold voltage.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a plot of threshold voltagechange (ΔVt) in mV versus time or period of stress (T_(stress)) inseconds shows the shifts in threshold voltage of a PFET device, with achannel length of 80 nm, as a function of time under a specific biascondition. A threshold shift 20 is under hot-carrier stress with highVgs (gate to source voltage) and Vds (drain to source voltage) biases,which induce a significant increase in threshold voltage. A shift 30 isunder NBTI stress with a high Vgs bias condition, which also causes aslight increase in the threshold voltage (note that the NBTI degradationin this particular technology/process is not significant). A shift 40 isunder the impact ionization condition mentioned above, which clearlyresults in a decrease, or recovery, in the threshold voltage of the PFETdevice under test.

Another method for employing CN recovery is to inject electrons from anN-well of a PFET device to its source and/or drain junctions by forwardbiasing these junctions. Under such condition, the electrons from theseforward biased PN junctions possess low energies and do not cause anydamage to the device. Note that this NBTI recovery by forward biasingN-well/source and/or N-well/drain junctions has been observed by thepresent inventors.

Referring to FIG. 2, I-V curves 60, 70 and 80 are shown for abody-contacted PFET on a silicon-on-insulator (SOI) substrate. Drain tosource current (I_(DS)) in Amps is plotted against gate voltage V_(g) involts. The I-V curves show the PFET response before stress (as marked bycurve 60), after NBTI stress (as marked by curve 70), and after therecovery process (as marked by curve 80). Observe that in curve 70, the1000 seconds of NBTI stress, with Vgs biased at −1.9V and other nodesgrounded as depicted by an inset 72, induces a threshold shift ΔVt of 10mV. By employing CN recovery (curve 80) with gate and source/drainbiased at +1.0V and +0.7V as depicted by an inset 82, the thresholdshift ΔVt is recovered by 80% to only 2 mV.

Referring to FIG. 3, a plot of threshold voltage change (ΔVt) in mVversus time in seconds for recovery time behavior of a 1.0 Volt PFET,having a channel length of 45 nm at 30 degrees C. is illustrativelyshown and compares two recovery conditions. A first recovery conditionplot 90 includes a 0.8V recovery, and a second recovery condition plot92 includes a 0.1V recovery.

It can be observed that the recovery at 0.1V under condition plot 92results in about 25% recovery in threshold shift after one second ofbias, which could be attributed to both NBTI self-thermal recovery andthe CN effect. Note that the CN effect with condition plot 90 results inas much as 50% threshold recovery within one second, and it alsocontinues to recover during the subsequent bias. Based on theseobservations, it is clear that many NBTI sensitive devices and circuitscan benefit from PN junction recovery effects through CN because CN isdamage-free and can be easily implemented in circuit designs.

Referring to FIG. 4A, a cross-sectional view illustratively showing asingle device recovery embodiment for the recovery of a damaged PFETdevice 100 is depicted. PFET device 100 includes a gate structure whichincludes a gate dielectric 109 and a gate conductor 111.

PFET device 100 includes device pads or nodes 105 of a source 120, adrain 140, a gate 130, and a device pad for a body contact 106 of anN-well or substrate 110. A body contact or substrate contact contacts,in this case, the N-well of substrate 110.

During normal device operation, voltages on the N-well node 106 tosource node 120 and/or drain node 140 are positively biased, which meansthat the corresponding N-well 110 and/or p+ junctions 122 and 124(source and/or drain) are reversely biased.

During a degradation repairing mode in accordance with the presentprinciples, the voltages between N-well pad 106 to source 120, drain 140and gate 130 are kept negative (e.g., at −0.7V to −0.8V), which resultsin slight forward biased PN junctions at source 120 and drain 140. Thispermits electron injection from N-well 110 into source 120 and/or drain140 regions which neutralizes trapped positive charge caused by NBTIeffects.

For a PBTI degraded NFET, a similar method can be applied. The NFETincludes polarities of the opposite type depicted in FIG. 4B. To addressPBTI effects, simply change bias voltage polarities, e.g., forward biasthe NFET P-well/drain and/or p-well/source junctions by applyingpositive voltage on the p-well.

Referring to FIG. 4B, a cross-sectional view illustratively showing asingle device recovery embodiment for the recovery of a damaged NFETdevice 150 is depicted. NFET device 150 includes a gate structure whichincludes a gate dielectric 109 and a gate conductor 111.

NFET device 150 includes device pads or nodes 155 of a source 170, adrain 190, a gate 180, and a device pad for a body contact 156 of aP-well or substrate 160. A body contact or substrate contact contacts,in this case, the P-well of substrate 160.

During normal device operation, voltages on the P-well node 156 tosource node 170 and/or drain node 190 are biased, which may mean thatthe corresponding P-well 160 and/or n+ junctions 172 and 174 (sourceand/or drain) are reversely biased.

During a degradation repairing mode in accordance with the presentprinciples, the voltages between P-well pad 156 to source 170, drain 190and gate 180 are kept positive, which results in slight forward biasedPN junctions at source 170 and drain 190. This permits hole injectionfrom P-well 160 into source 170 and/or drain 190 regions whichneutralizes trapped negative charge caused by PBTI effects.

Referring to FIG. 5, a CMOS logic circuit repair is described. A CMOSinverter circuit 200 includes an input pad 210, an output pad 220, aPFET N-well contact pad 250 and an NFET source pad 230. During normalCMOS operation, pad 230 is connected to Vss or ground, and pad 250 isconnected to high voltage, e.g., Vdd.

Referring to FIG. 6, a timing diagram shows corresponding repair modewaveforms (voltages) at pads 210, 230 and 250 as indicated by V210,V230, and V250, respectively. In a repair mode for a PMOS device 205,input pad 210 is set high (Vdd), pad 230 is switched from Vss to Vdd,while pad 250 is biased at Vdd-Vfw (e.g., Vfw=0.7V to 0.8V). Vfw is theforward PN junction voltage, and 0.7V˜0.8V is the turn-on voltage of thePN junction. In the repair mode, both PN junctions of the PFET sourceand drain are slightly forward biased.

In this case with continued reference to FIGS. 5 and 6, the ground lineis fed with the same voltage as power supply, Vdd, so that when a gate207 is switched from low to high, the output node 220 will also risefrom low to high which is now applied to the drain of the PMOS device205. This will make the PMOS device 205 to be efficiently fixed.

A similar repairing method can be applied for NMOS 215 by switching theNFET P-well 260 to Vfw while input 210 is kept low or 0V, and byswitching pad 270 to ground while pad 230 is grounded as in normaloperation. Repairing a circuit should facilitate proper bias at eachnode of the device so that the unwanted charge accumulated in the gateregion can be eliminated.

Referring to FIG. 7, an SRAM array repair will now be illustrativelydescribed in accordance with another embodiment. An illustrativesix-transistor SRAM cell 300 includes a pair of pull-up PMOS devices, P0and P1; a pair of pull-down NMOS devices, N0 and N1 and a pair of NMOStransfer devices, T0 and T1. Gates of the transfer devices T0 and T1 aretied to a wordline WL. Drains of each transfer device T0 and T1 are tiedto a left bitline LBL and a right bitline RBL, respectively. A body ofP1 is tied to left n-well contact LNW, while a body of P0 is tied toright n-well contact RNW. A power supply for the cell 300 is tied toVdd. A ground line 302 is tied to Vss in a normal mode when not in a fixmode (or when FIX bar enables conduction through a transistor T3). Theground line 302 is tied to Vdd when is in the fix mode (or FIX enabletransistor T2 to conduct). Two transfer gates T2 and T3 control thevoltage on ground line 302.

The cell 300 is preferably designed to have only the pull-up PMOSdevices P0 and P1 fixed after their Vt shifts exceed a predeterminedlevel. A similar arrangement can be implemented to fix the pull downNMOS devices, N0 and N1. During the repair mode, FIX=1 which turns on T2and turns off T3 and thus connects the ground line to Vdd. The bodies ofthe P0 and P1 devices are alternatively biased during the repairingperiod. Each time, all the left pull-up PMOS devices (in this case P1)in the array are repaired simultaneously. When this is done, all theright pull-up PMOS devices (in this case P0) are fixed subsequently byproperly biasing each bitline (LBL and RBL) of the paired bitlines.

Referring to FIG. 8, a simplified SRAM cell 300 is shown, wherein thecell 300 comprises a power supply Vdd, a ground Vss, a Wordline WL, aright bitline RBL, a left bitline LBL, and two N-well contacts LNW andRNW. The cell 300 is employed for constructing a 4×4 array (402) andassociated repair circuits are shown in FIG. 9.

Referring to FIG. 9, a repair circuit 400 for repairing a 4×4 array 402of memory cells 300 is employed as an example to indicate how an SRAMarray can be constructed in accordance with the present principles. Therepair circuit 400 provides that pull up PMOS devices (P0 and P1, inFIG. 7) can be periodically fixed for NBTI related determination. A fixcontrol (FC) circuit 406 is employed to generate a left side fix andright side fix signal, or FIX_L and FIX_R, respectively. These twocontrol signals are connected to a NOR gate 408 and an OR gate 410.

When neither side fix is called, an output of the NOR gate 408 willconnect a ground line(s) 412 of the SRAM array to Vss. When either sideis in fix mode, the ground line(s) 412 of the SRAM array is connected toVdd. The connection of ground lines 412 is selectively performed throughtransfer devices T11, T12, T13 and T14 using the outputs of the NOR gate408 and the OR gate 410.

During left-side fix mode, the FIX_L is high, the left sides of all thecells 404 are under repair. At that time, WL lines (WL0, WL1, etc.) canbe simultaneously or sequentially switched on to access each cell 404.Left side bitlines (LBL0, LBL1, etc.) and left side n-wells (NW0, NW1,NW2, etc.) are switched on via transfer devices T16, T19 and T15, T18,respectively. The left-side n-wells are tied to FIX_NW level. Theleft-side pull-up PMOS devices (P0, P1, shown in FIG. 7) of all thecells 404 are under repair. The repair can be performed in a fixed timeor using a monitoring device (not shown) to determine when the repair iscomplete.

A cell could be monitored by sampling write and read operations. Thefailing of either operation indicates the need of repair. Alternately, amonitoring device, e.g., a separate PFET, under similar SRAM PFETconditions and operations may be employed with a threshold voltagemonitoring circuit. If the PFET threshold voltage increases to a presetvalue, the cell repairing mode can be turned on. The threshold voltagemay be compared to the preset value or determined from storage (e.g., alook-up table or the like).

When the left devices are fixed, the right side devices are repairedsubsequently by employing transfer devices T17, T18, T19 and T20. Thesedevices permit access to N-wells and right bitlines (RBL0, RBL1, etc.).Based on the same principles, numerous array arrangements may beemployed to achieve the same or similar results within the scope of thepresent invention.

Referring to FIG. 10, a fix control (FC) circuit 406 includes a sensor502, wherein at least one monitor device (not shown) is provided, and athreshold level of the monitor device is compared to a reference levelto determine if a repair or reset is needed for a device or devicesbeing monitored. The monitor device is preferably under the same stressas the device or devices being monitored (e.g., the devices in the cells404 of the SRAM array). When the threshold level exceeds that of thereference level, a fix signal is triggered by circuit 406. For example,FIX_L and/or FIX_R may be generated.

Since a fix is preferably not executed when the array is under normaloperation, an enable signal 501 is provided by the system to time whenit would be appropriate to repair devices. When the array is in a fixmode, the sensor 502 decides whether the array needs to be fixed or not.If a repair is needed, a trigger signal will activate a left arrayrepair block 504 by issuing a FIX_L control signal to the array. Atleast one monitor device of sensor 502 may be employed to time a fixperiod. When the threshold level of the damaged device is recovered to apredetermined value, the fix is complete.

Then, a right array repair block 506 is activated and issues a FIX_Rsignal to the array. When all the devices are fixed, a finish repairblock 508 ends the fix cycle and informs the system (DONE) that thearray is ready for a normal function mode. It should be noted that theorder in which the left or right side of the array are repaired may beswitched. In one embodiment, a single fix signal may be output to test asingle device or component.

Referring to FIG. 11, I-V curves 560, 570 and 580 are shown for a PFETon a silicon-on-insulator (SOI) substrate without body contact. Drain tosource current (I_(DS)) in Amps is plotted against gate voltage V_(g) involts. The I-V curves show the PFET response before stress (as marked bycurve 560), after NBTI stress (as marked by curve 570), and after therecovery process (as marked by curve 580). Observe that in curve 570,the 1000 seconds of NETI stress, with Vgs biased at −2.1V and othernodes grounded as depicted by an inset 572, induces a threshold shiftΔVt of 11 mV. By employing CN recovery (curve 580) with gate andsource/drain biased respectively at +1.1V and +0.0V as depicted by aninset 582, the threshold shift ΔVt is recovered by 10% to only 10 mV.

Referring to FIG. 12, a CMOS logic circuit repair is described for afloating body SOI CMOS. A CMOS inverter circuit 502 includes an inputpad 510, an output pad 520, and source pads 500 (for PFET) and 530 (forNFET). During normal CMOS operation, pad 530 is connected to Vss orground, and pads 500 and 530 are switched between high voltage, e.g.,Vdd, and ground, e.g., Vss, as indicated in FIG. 13.

A method for repairing degraded field effect transistors includesforward biasing PN junctions of one of a source and a drain of a fieldeffect transistor (FET) and accumulating charge in a gate region toneutralize charge in the gate region. The charge is preferablyaccumulated by applying one of a supply voltage and a ground voltage toa source pad of the device being repaired to accumulate charge in thegate region.

Referring to FIG. 13, a timing diagram shows corresponding repair modewaveforms (voltages) at pads 510, 500 and 530 as indicated by V510,V500, and V530, respectively. In a repair mode for a PMOS device 504(FIG. 12), input pad 510 is set high (Vdd), pad 500 is switched from Vddto Vss, while pad 530 is maintained at Vss. In the repair mode, the NBTIdegraded PFET (PMOS) is biased into accumulation mode to partiallyrecover the NBTI degradation without body contact (floating body).

A similar repairing method can be applied for NMOS 506. Repairing acircuit should facilitate proper bias at each node of the device so thatthe unwanted charge accumulated in the gate region can be neutralized.

Referring to FIG. 14, an SRAM array repair will now be illustrativelydescribed in accordance with the floating body embodiment. Anillustrative six-transistor SRAM cell 300 includes a pair of pull-upPMOS devices, P0 and P1; a pair of pull-down NMOS devices, N0 and N1 anda pair of NMOS transfer devices, T0 and T1. Gates of the transferdevices T0 and T1 are tied to a wordline WL. Drains of each transferdevice T0 and T1 are tied to a left bitline LBL and a right bitline RBL,respectively.

A power supply 602 for the cell 300 is tied to Vdd and Vss usingtransistors T21 and T22, respectively. A ground line 604 is tied to Vss.In a fix mode, a FIX signal or FIX bar signal appropriately ties thepower line 602 to Vss or Vdd by enabling conduction through transistorT21 or T22.

The pull-up PMOS devices P0 and P1 are fixed after their Vt shiftsexceed a predetermined level. A similar arrangement can be implementedto fix the pull down NMOS devices, N0 and N1. During the repair mode,T22 is turned on and turns off T21 to create the situation depicted inFIG. 13 for P0 and P1. The bodies of the P0 and P1 devices are floatingin this embodiment during the repair period. Each time, all the leftpull-up PMOS devices (in this case P1) in the array are repairedsimultaneously. When this is done, all the right pull-up PMOS devices(in this case P0) are fixed subsequently by properly biasing eachbitline (LBL and RBL) of the paired bitlines.

Having described preferred embodiments of self-repair integratedcircuits, devices and repair methods (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeand spirit of the invention as outlined by the appended claims. Havingthus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A method for repairing field effect transistorsin a memory circuit having an array of memory cells, comprising: forpull-down N-type devices, switching a supply line from a supplypotential to a ground potential; switching on all wordlines in thearray; repairing N-type devices on a first side by connecting a P-wellof the N-type devices to a voltage greater than the ground potential,but less than the supply potential; deactivating bitlines on the firstside; repairing N-type devices on a second side by connecting the P-wellof the N-type devices to the voltage greater than the ground potential,but less than the supply potential; and deactivating bitlines on thesecond side.
 2. The method as recited in claim 1, wherein the groundpotential is Vss and the voltage greater than the ground potential, butless than the supply potential is Vss+Vfw, where Vfw is a forward biasvoltage.
 3. The method as recited in claim 1, further comprisingtriggering a fix mode when a threshold voltage exceeds a reference levelin at least one device to be repaired.
 4. The method as recited in claim3, further comprising generating at least one control signal to initiatedevice repairs.
 5. The method as recited in claim 3, further comprisingresuming normal operation when all repairs have been performed.
 6. Themethod as recited in claim 1, further comprising triggering a fix modewhen any one of write and read operations fails.